1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, copier or facsimile machine.
2. Related Background Art
As described in Japanese Patent Application Laid-Open No. H11-258931, the methods described hereunder are conventionally known as methods that adjust image processing characteristics (hereunder, referred to as “image control method”) in image forming apparatuses such as printers, copiers and facsimile machines.
According to one method, after an image forming apparatus is turned on and warm-up operations are completed, a specific pattern is formed on an image bearing member such as a photosensitive drum. The density of the formed pattern is then read, and based on the obtained density value, operations of circuits that determine the image forming conditions, such as a γ correction circuit (gamma correction circuit), are changed to stabilize the quality of formed images.
According to another method, when the gradation characteristics of an image forming apparatus have changed due to fluctuations in environmental conditions, the image quality can be stabilized in accordance with the fluctuations in the environmental conditions by forming a specific pattern on an image bearing member once more and reading the density value, and providing feedback again to circuits that determine the image forming conditions such as a γ correction circuit.
Methods are also known which carry out the above-described control for each image forming operation or at the end of each image forming operation to ensure better stabilization.
Further, when an image forming apparatus has been used over a long period, there are cases in which the density that has been read for a pattern on an image bearing member does not match with the density of an image which is actually printed. Therefore, a method is known in which a specific pattern is formed on a recording material and the image forming conditions are then corrected on the basis of the density value thereof.
A method is also known which corrects a gamma look-up table (γLUT) on the basis of density information for one image pattern, creates a γLUT modulation table, and adds correction information that had been lacking in a gamma correction circuit.
Since the control in the afore-mentioned methods involves time and working operations, the image control cannot be carried out frequently. Accordingly, it cannot be said that image quality such as gradation reproduction and the like can be stabilized sufficiently with respect to the imaging characteristics of image forming apparatuses that vary from one minute to the next.
Further, in a method which enables correction of a gamma correction circuit to be conducted comparatively simply by correcting a γLUT based on density information of one image pattern and then adding the correction information to a gamma correction circuit, when the number of additions increases the gradation differences in the γLUT can no longer be disregarded, and thus false contour is generated.
In addition, when increases in potential in an exposure portion vary several dozen volts for several sheets of formed images as a result of accumulation of residual charges on a photosensitive member caused by exposure, even when a method is adopted which detects the densities of patches having a halftone density that are formed in a non-image formation area (non-image forming area) and corrects a γLUT at a high frequency based on the detected values, since it is necessary to set the correction of the γLUT on the premise of a certain degree of stability in the potential, it is not possible to maintain a stable image density and color tint.